Learning from the past : how low [CO2] studies inform plant and ecosystem response to future climate change

Atmospheric [CO2] over the past 800 000 yr has varied generally as a function of glacial periods, with minima (c. 170–200 ppm) during glacial periods and maxima (c. 280–300 ppm) during inter-glacial periods (Luthi et al., 2008). During the Last Glacial Maximum (LGM; 18,000–20,000 yr ago), atmospheric [CO2] ranged from 180 to 200 ppm, which is approximately half the current [CO2] (392 ppm), and among the lowest [CO2] observed during the evolution of vascular land plants over the past 350 million yr. While it has been observed that low atmospheric [CO2] directly limits photosynthesis, with subsequent reductions in biomass production, reproduction, and survival, these studies have primarily been conducted on modern plants grown for a single generation in low [CO2]. Subsequently, they do not address the potential evolutionary adaptive responses to low [CO2] which would only become evident in plants growing for long-time periods and many generations under these environmental conditions. In a fascinating study, in this issue of New Phytologist, Gerhart et al. compared stable carbon isotope ratios found in the annual rings of glacial Juniperus wood preserved in the La Brea tar pits in southern California with modern Juniperus wood in the nearby mountains, and used them to calculate ci/ca over the 50,000-yr period spanning the last glacial period to modern times.